Over the past twenty years, nuclear magnetic resonance imaging (MRI) has developed into an important modality for both clinical and basic-science imaging applications. A large portion of MRI techniques are based on spin-echo (SE) acquisitions because they provide a wide range of useful image contrast properties that highlight pathological changes and are resistant to image artifacts from a variety of sources such as radio-frequency or static-field inhomogeneities.
Spin-echo-based methods can be subdivided into two categories, including those that generate one spin echo for each desired image contrast following each excitation radio-frequency (RF) pulse, and those that generate more than one spin echo for each desired image contrast following each excitation RF pulse. The first category includes, but is not limited thereto, the techniques commonly referred to as “conventional SE” imaging. The second category includes, but is not limited thereto, a method called “RARE” (See Hennig J., Nauerth A., Friedburg H., “RARE Imaging: A Fast Imaging Method for Clinical MR”, Magn. Reson. Med. 1986, 3:823-833; and Mulkern R. V., Wong S. T. S., Winalski C., Jolesz F. A., “Contrast Manipulation and Artifact Assessment of 2D and 3D RARE Sequences”, Magn. Reson. Imaging 1990, 8:557-566, of which are hereby incorporated by reference in their entirety) and its derivatives, commonly referred to as “turbo-SE” or “fast-SE” imaging (See Melki P. S., Jolesz F. A., Mulkern R. V., “Partial RF Echo Planar Imaging with the FAISE Method. I Experimental and Theoretical Assessment of Artifact”, Magn. Reson. Med. 1992, 26:328-341 and Jones K. M., Mulkern R. V., Schwartz R. B., Oshio K., Barnes P. D., Jolesz F. A., “Fast Spin-Echo MR Imaging of the Brain and Spine: Current Concepts”, AJR 1992, 158:1313-1320, of which are hereby incorporated by reference in their entirety). For the purposes of this disclosure, we are primarily interested in the generalized form of techniques in the second category; however the present invention is applicable to the first category as well. The term “generalized” refers to the form of the spatial-encoding gradients that are applied following any given refocusing RF pulse. For example, RARE imaging encodes one line of spatial-frequency space (k-space) data following each refocusing RF pulse using a constant, frequency-encoding magnetic field gradient. In contrast, “GRASE” imaging (See Feinberg D. A., Oshio K. “GRASE (Gradient- And Spin-Echo) MR Imaging: A New Fast Clinical Imaging Technique”, Radiology 1991, 181:597-602; and Oshio K., Feinberg D. A. “GRASE (Gradient- And Spin-Echo) Imaging: A Novel Fast MRI Technique”, Magn. Reson. Med. 1991, 20:344-349, of which are hereby incorporated by reference in their entirety) encodes three or more lines of k-space data following each refocusing RF pulse using an oscillating, frequency-encoding gradient waveform. This oscillating gradient waveform collects one line of k-space data that includes the spin echo, and one or more additional lines of k-space data before the spin echo and after the spin echo. One skilled in the art would appreciate that there exist an infinite number of possibilities for spatially encoding the MR signal following each refocusing RF pulse. For the purpose of this disclosure, we define the term “spin-echo-train” imaging to encompass all of these possibilities, including, but not limited thereto, RARE, turbo-SE, fast-SE and GRASE imaging, because the present invention deals with, among other things, the RF-pulse history during the echo train, not the details of the spatial encoding.
In general, one of the major goals of technique development for MRI has been to increase the amount of k-space data sampled per unit time, under the constraints of obtaining the desired image contrast and maintaining image artifacts at a tolerable level. Increases in the data rate are typically traded for a decrease in the image acquisition time and/or an increase in the spatial resolution. In this respect, spin-echo-train methods have played an important role; fast-SE imaging is routinely and widely used in clinical MRI.
For instance, the echo trains used in clinical fast-SE imaging generally employ high flip angles (>100°) for the refocusing RF pulses, and their durations are typically less than the T2 relaxation times of interest for short effective echo times (e.g., T1 or proton-density weighting) or less than two to three times these T2 values for long effective echo times (e.g., T2 weighting or “FLAIR”; see Hajnal J. V., Bryant D. J., Kasuboski L., Pattany P. M., De Coene B., Lewis P. D., Pennock J. M., Oatridge A., Young I. R., Bydder G. M., “Use of Fluid Attenuated Inversion Recovery (FLAIR) Pulse Sequences in MRI of the Brain”, J. Comput. Assist. Tomogr. 1992, 16:841-844, of which is hereby incorporated by reference in its entirety). For example, considering brain imaging at 1.5 Tesla, these limits translate to echo-train durations of <100 ms and <300 ms for short and long effective echo times, respectively. When high flip angles are used for the refocusing RF pulses, echo-train durations that are longer than these limits can substantially degrade image contrast and introduce artifacts such as blurring (See Mulkern et al.; Melki et al.; Constable R. T., Gore J. C., “The Loss of Small Objects in Variable TE Imaging: Implications for FSE, RARE and EPI”, Magn. Reson. Med. 1992, 28:9-24; and Ortendahl D. A., Kaufman L., Kramer D. M., “Analysis of Hybrid Imaging Techniques”, Magn. Reson. Med. 1992, 26:155-173, of which are hereby incorporated by reference in their entirety).
Nonetheless, if it were possible to substantially lengthen echo-train durations beyond these limits, while achieving the desired image contrast and limiting artifacts, it would represent a useful and widely applicable advance.
Preliminary studies with the goal of lengthening the echo-train duration in spin-echo-train-based acquisitions have been performed by other researchers. Over a decade ago, Hennig (See Hennig J., “Multiecho Imaging Sequences with Low Refocusing Flip Angles”, J. Magn. Reson. 1988, 78:397-407, of which is hereby incorporated by reference in its entirety) proposed the use of constant, low-flip-angle refocusing RF pulses to introduce a T1 dependence to the evolution of the echo train and thereby lengthen its usable duration. More recently, this concept was extended by Alsop, who derived variable flip-angle series based on the “pseudosteady-state” condition of a constant signal level when T1 and T2 relaxation are neglected (See Alsop D. C., “The Sensitivity of Low Flip Angle RARE Imaging”, Magn. Reson. Med. 1997; 37:176-184, of which is hereby incorporated by reference in its entirety). Alsop also found that the echo-train performance was improved by using a signal evolution that decreased for the first few echoes and was then constant, instead of being constant for the complete echo train. Using these evolutions, artifact-free human brain images with T2-weighting were acquired by Alsop. An 80-echo train with a duration of 400 ms and asymptotic flip angles ranging from 17° to 90° were used.
Turning to the present invention, a method and related apparatus is provided for lengthening the usable echo-train duration for spin-echo-train imaging substantially beyond that achievable with the constant, low-flip-angle or pseudosteady-state approaches. The present invention method and apparatus explicitly consider the T1 and T2 relaxation times for the tissues of interest and thereby permit the desired image contrast to be incorporated into the tissue signal evolutions corresponding to the long echo train. Given the considerable role that spin-echo-train methods already play in MR imaging, the present invention methodology will be of significant importance.